Problem:
I created a new post type, added custom fields but when I try to create a new form I get a Critical site error. I can open/edit existing forms, but cannot create new forms.
Solution:
This issue has been fixed now with the Types version 3.4.20, please update it by going to Plugins -> Add New -> Commercial tab. Alternatively, you can download it from here: https://toolset.com/account/downloads/, and upload directly into your Plugins -> Add New page.